Vacuum switching devices which are designed to switch large currents have been produced for many years. Vacuum switching devices, generally consist of an evacuated envelope which includes an insulating component, a fixed electrode assembly and a moving electrode assembly. The two electrodes are engaged and disengaged mechanically to perform the switching function. It is necessary for this movement to occur without breaking the seal of the evacuated envelope, which is normally achieved by means of a bellows or diaphragm arrangement. The electrodes are made of a metal with good electrical conductivity such as copper. The contact faces are in the form of discs made of special material best able to withstand erosion by the arc which forms as the contacts are separated.
As the electrodes separate in switching, an electric arc forms. Due to aspects of the physics of the vacuum arc, the arc is diffused over the contact surfaces at low currents but becomes constricted to a small area or areas of the contact surfaces at a current of a few thousands of amps. It is necessary to control this constriction of the arc, which would inhibit successful interruption of the current due to overheating of points on the contact surfaces. This is achieved by constraining the current flow leading up to the contact surfaces to follow a helical path such that the magnetic field associated with the current influences the arc in a desired way. In one known form the magnetic field is generally radial over the contact surfaces, and in another known form the field is generally spread over the contact area and in the direction of the axis of the assembly. This invention concerns the axial field form of contacts.
A number of different designs of axial field electrodes and electrode assemblies have been proposed. Examples of such designs may be seen in EP 0349303, DE 3915519, DE 3610241, GB 2 338 1111 1 1, and GB2174843. The structure of a known axial field contact is shown in FIG. 3. In this design the contact head of the electrode is cup-shaped, formed from a cylindrical wall part (6) brazed to a disc shaped base (7). The wall part has a plurality of slots (8) formed in a generally helical direction. The wall part of the cup supports a contact disc (5). When current flows through the electrodes and an arc is formed between them, the slots constrain the current to take a helical path around the axis of the assembly, whereby a significant magnetic field is generated in a direction parallel to the axis of the contacts. This axial field acts to diffuse an arc that would otherwise be constricted and so enables higher currents to be interrupted. In some recent designs slots are cut into the base of the contact cup as well.
A disadvantage of these design is this type of design is that the slots mechanically weaken the contact assembly and so some means of mechanical support may be necessary to help support the contact disc and prevent mechanical distortion of the contact. In FIG. 3 this support takes the form of a cylinder of strong metal (9) of relatively poor electrical conductivity, which prevents collapse of the slotted outer part of the contact. Another approach to the problem is to allow the distortion to occur, and the slots to partially close, but to allow for the consequent shortening of the contact cup in the design and use of the vacuum switching device.